1. Field of the Invention
The present invention relates to an electromechanical transducer, such as a capacitive electromechanical transducer, which performs the transmission and reception of elastic waves, including ultrasonic waves (the term “transmission and reception” in the present description means at least one of the transmission and the reception).
2. Description of the Related Art
For the purpose of transmitting and receiving ultrasonic waves, a capacitive micromachined ultrasonic transducer (CMUT), which is a capacitive ultrasonic transducer, has been proposed. The CMUT is fabricated by a microelectromechanical systems (MEMS) process, which uses a semiconductor process. FIG. 10 is a schematic sectional view of an array CMUT in Knight J, McLean J, and Degertekin F L, “Low temperature fabrication of capacitive micromachined ultrasonic immersion wave transducers on silicon and dielectric substrates”, IEEE Trans. Ultrason., Ferroelect., Freq. Contr. Vol. 51, No. 10, pp. 1324-1333), 2004. In FIG. 10, reference numeral 101 denotes a vibrating film, reference numeral 102 denotes a first electrode (upper electrode), reference numeral 103 denotes supporting portions, reference numeral 104 denotes a gap, reference numeral 105 denotes a second electrode (lower electrode), reference numeral 106 denotes a substrate, and reference numeral 107 denotes an insulating film. The first electrode 102 is deposited on the vibrating film 101, and the vibrating film 101 is disposed on the substrate by being supported by the supporting portions 103 formed on the substrate 106. The second electrode 105 is disposed on the substrate 106 at a position where it opposes the first electrode 102 on the vibrating film 101 with the gap 104 (normally ranging from tens of nm to hundreds of nm) provided therebetween. The constitution comprising the vibrating film 101 and the first and the second electrodes opposing each other with the gap 104 provided therebetween is defined as one set and referred to as a cell 200. Either the first electrodes or the second electrodes are electrically interconnected and have a common potential. The electrodes sharing the common potential are referred to as common electrodes. In this case, a description will be given of a constitution in which the second electrodes 105 are the common electrodes. The second electrodes (common electrodes) 105 are connected by a wire 108 to a potential difference setter 121 capable of applying a desired potential, and a predetermined DC potential difference is set between the second electrodes 105 and the opposing first electrodes 102. Of the first and the second electrodes, the electrodes that are not the common electrodes are electrically connected for each given cell group and carry the same potential. The given cell group is referred to as an element 201, which indicates the unit of device that transmits and receives elastic waves. In the following description, the electrodes carrying the same potential for each cell group (element) will be referred to as signal electrodes. In this case, the description will be given of a configuration in which the first electrodes 102 are the signal electrodes. The first electrodes (signal electrodes) 102 of each element are connected to a driving and detecting unit 122 by a wire 109. The insulating films 107 are deposited on the substrate 106 to provide insulation between the substrate 106 and the wires, so that the wiring between the signal electrodes of different elements or the wiring of the common electrodes are electrically isolated.
At least either a transmitting operation or a receiving operation can be accomplished by operating the driving and detecting unit 122. The transmitting operation is an operation in which the driving and detecting unit 122 generates an AC voltage and applies the AC voltage to the first electrodes (the signal electrodes) so as to generate an AC electrostatic attractive force between the first and the second electrodes 102 and 105, thereby vibrating the vibrating films 101 formed integral with the first electrodes 102 to transmit an elastic wave to the outside. Meanwhile, the receiving operation is an operation to receive an elastic wave, which vibrates the first electrodes 102 formed integral with the vibrating films 101, thereby detecting the magnitude of a received elastic wave. More specifically, the capacitance between the first and the second electrodes 102 and 105, respectively, changes due to the vibration of the vibrating films 101, and the magnitude of a current generated by a changing electric charge induced in the first electrodes (the signal electrodes) is detected by the driving and detecting unit 122 so as to detect the magnitude of the elastic wave.
In the configuration described above, an element as the device unit for transmitting and receiving elastic waves depends on the area in which the signal electrodes are electrically connected, so that the shape of the element cannot be changed. On the other hand, in the case of transmitting and receiving elastic waves, the optimal shape of the element varies according to an application for which the element is used (e.g., the measurement of the elastic waves of different objects). For this reason, it is not easy to use an electromechanical transducer having an element with a fixed shape for different applications.